Hazop Guide Words: LESS
HAZOP guide words and deviations — IEC 61882:2016 guide words applied to common process parameters across node types (pipe, vessel, reactor, heat exchanger, pump, compressor, control loop, batch step), with typical causes, consequences, and safeguards
| guide word | deviation key | context | deviation description | likelihood class | node type | process parameter | severity class | typical causes | typical consequences | typical safeguards |
|---|---|---|---|---|---|---|---|---|---|---|
| LESS | flow#control_loop#general | general | Control valve fails closed — flow lower than setpoint | possible | control_loop | flow | minor | Valve actuator failure; instrument air failure (fail-closed valve); controller output saturated low; positioner failure; plugged valve | Process starvation; heating/cooling loss; overconcentration; product quality degradation | Low-flow alarm; valve position feedback; controller deviation alarm; fail-safe valve position specification |
| LESS | flow#control_loop#plugged_valve | plugged_valve | Low flow through control valve due to partial plugging | likely | control_loop | flow | minor | Fouling or scale buildup on valve trim; polymer deposition; crystallization in valve body; solids carryover in process fluid | Process upset due to insufficient flow; temperature or pressure deviation; product quality degradation; heat exchanger undersupply | Valve differential pressure monitor; upstream strainer; regular valve inspection and cleaning; valve Cv sizing review |
| LESS | flow#pipe#general | general | Flow rate lower than design intent | likely | pipe | flow | minor | Partial blockage of filter/strainer; control valve partially closed; pump degradation; cavitation; line fouling; upstream pressure drop | Insufficient cooling or heating; process concentration upset; product quality degradation; cavitation damage to pump | Low-flow alarm (FAL); differential pressure indicator across strainer; pump vibration monitor |
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